In a gas turbine, a predetermined clearance is provided between a tip of the rotor blade and the shroud that faces the tip of the rotor blade, so that the tip of the rotor blade and the shroud do not come in contact with each other during operation. If this clearance is too large, combustion gas leaks from the pressure surface side to the suction surface side of the rotor blade, thereby the combustion gas that can be used for driving the turbine decreases. As a result, the operation efficiency of the gas turbine decreases. Therefore, the clearance is set as small as possible, for suppressing the leak of the combustion gas as much as possible, to improve the performance of the gas turbine.
However, if the clearance is too small, in the initial stage of startup of the gas turbine, the tip of the rotor blade and the shroud may slide with each other, resulting from thermal expansion of the rotor blade, eccentricity of a turbine rotor, vibrations occurring in the whole gas turbine, or the like (a so-called initial sliding). When the gas turbine is operated for long time, the shroud exposed to the high-temperature gas gradually causes a thermal deformation, thereby the tip of the rotor blade and the shroud may slide with each other (a so-called secondary sliding).
In general, the shroud comprises a coating as a thermal insulation or antioxidation on the internal peripheral face thereof. For example, a thermal barrier coating (TBC) may be provided for thermal insulation, or an antioxidant coating consisting of McrAlY may be provided, where M is one or more of iron, nickel, and cobalt. These coatings often have high hardness, and hence, if the tip of the rotor blade and the internal peripheral face of the shroud slide with each other, the rotor blade may be largely damaged.
Japanese Patent Application Laid-Open Nos. 4-218698 and 9-504340, and U.S. Pat. No. 5,702,574 disclose a rotor blade having an abrasive coating, in which abrasive particles are dispersed in a matrix of McrAlY, which is an antioxidant material. In this rotor blade, for example, cubic boron nitride (CBN) or the like is used as the abrasive particles. The cubic boron nitride is a material having high hardness, and hence, if the rotor blade and the internal peripheral face of the shroud slide with each other, the abrasive particles comprising this cubic boron nitride polish the internal peripheral face of the shroud. As a result, appropriate clearance can be maintained between the rotor blade and the shroud.
This abrasive coating may be formed as follows. That is, abrasive particles are temporarily fixed to the rotor blade body, and a matrix is formed around the abrasive particles by electrodeposition. In other words, the matrix is formed by the growth of a deposit. Since the growth of the deposit requires time, this forming method has poor efficiency. Further, the formation of the matrix by the electrodeposition is generally expensive. Further, electrodeposition needs large-scale equipment, and it is difficult to newly build the electrodeposition equipment from a standpoint of environmental protection.
Japanese Patent Application Laid-Open No. 10-30403 discloses an abrasive coating formation method in which a matrix is formed by a thermal spraying method. The thermal spraying method is a method of allowing a metal layer to grow by injecting a molten metal, and has a feature in that it is highly efficient as compared with the electrodeposition method. In the thermal spraying method, however, when abrasive particles are temporarily fixed to the rotor blade body, the electrodeposition method is used. Therefore, it has the problems described above, and it is also difficult to accurately control the thickness of the matrix, and large-scale thermal spraying equipment is required. When abrasive particles such as cubic boron nitride are dispersed in the metal matrix by the thermal spraying method, since the abrasive particles are buried in the molten metal, it is necessary to remove the molten metal until the abrasive particles are exposed. However, it is difficult to expose the abrasive particles, and hence, it becomes difficult for the abrasive particles to polish the internal peripheral face of the shroud. Further, the metal matrix may be welded on the internal peripheral face of the shroud, to damage the rotor blade.
An antioxidant coating of the TBC or MCrAlY may be formed on the internal peripheral face of the shroud. These coatings are generally formed by the thermal spraying method, such as an atmospheric plasma spray (APS) method, a high velocity oxygen fuel (HVOF) method, a low pressure plasma spray (LPPS) method, or a detonation gun (D-GUN) method.
It is an object of the present invention to provide a coating formation method, a coating formation material, an abrasive coating formation sheet, a rotor blade in a gas turbine, on which an abrasive coating or the like is formed by the coating formation method, and a gas turbine using this rotor blade.